1. Field of the Invention
This invention relates to methods and apparatus for detecting and locating changes in variables.
2. Introduction to the Invention
Copending commonly assigned U.S. Application Ser. Nos. 832,562 filed Feb. 20, 1986, 831,758 filed Feb. 20, 1986, now abandoned in favor of continuation-in-part application Ser. No. 017,375, and 838,725 filed Mar. 11, 1986, the entire disclosures of which are incorporated herein by reference, disclose methods and apparatus for detecting and obtaining information about (particularly locating) changes in variables. Such changes are referred to in that application, and likewise in this application, by the generic term "event". In a preferred embodiment disclosed in Ser. No. 832,562 (and also disclosed in Ser. No. 618,109, filed June 7, 1984, now abandoned, of which Ser. No. 832,562 is a continuation-inpart), the apparatus comprises a sensor cable comprising two insulated wires and two "non-insulated" wires which are not connected to each other in the absence of an event but which become electrically connected to each other upon occurrence of an event. The non-insulated wires can for example comprise a metal core surrounded by a conductive polymer jacket, so that the wires become electrically connected if there is a water leak. The term "conductive polymer" is used herein to denote a composition which comprises a polymeric component (e.g. a thermoplastic, or an elastomer, or a mixture of two or more such polymers) and, dispersed in the polymeric component, a particulate conductive filler (e.g. carbon black, graphite, a metal powder, or two or more of these). The possibility of connection can for example exist at all points along an elongate path or over selected stretches or discrete locations of an elongate path; for example the non-insulated wires can be wires which are insulated except at spaced locations at each of which they can become connected through a switch which is switched on by occurrence of an event at the location of that switch.
When a four-wire sensor cable of the kind just described is used to detect events along one or more branch paths extending from a main (or trunk) path, the cable can simply be routed along the branch path to the end of the branch path and then back again to continue down the trunk path. However, this can lead to misleading results as to the location of the event, since an event taking place along the branch can cause connection of the non-insulated wires in both the outgoing branch cable and the incoming branch cable. This problem can be overcome by running the sensor cable from the junction of the trunk and branch paths to the end of the branch path only, and using four insulated "jumper" wires to make the appropriate connections from the end of the branch back to the trunk; but this is an expensive and inconvenient solution, especially when space is limited, eg. in a double containment system.